(a) Field of the Invention
The present invention is related to a flat panel antenna, and more particularly, to one that is capable of precise control of the impedance of the flat panel antenna with a value of normalized impedance approaching 1, i.e., the antenna impedance approaches 50Ω of the system impedance to upgrade radiation efficiency of the flat panel antenna.
(b) Description of the Prior Art
Advanced communication technology has expands its applications in technology products; and communication products also tends to be more diversified making the design and study of antenna even critical in determination of the transmission and receiving quality. The antenna relates to a device to either radiate or receive electron magnetic (EM) waves and generally the features of an antenna can be made clear by its work frequency, radiation pattern, return loss and antenna gain parameters.
A flat panel antenna of the prior art as illustrated in FIG. 1 of the accompanying drawings includes a rectangular radiation metal member 11 as the primary radiation portion of the antenna; a flat ground plane 12; a substrate 14 disposed between the radiation metal member 11 and the ground plane 12; and a feeding device 13 comprised of a feeding cable penetrating through the ground plane 12 to feed upright into the radiation metal member 11 through a metal probe for transmitting signals.
FIG. 2 shows experimental data regarding return loss of the antenna (with a design central frequency at 915 Hz). Wherein Curve A indicates the measured bandwidth of the flat panel antenna; and the return loss of the central frequency of the antenna is approximately −14 dB (referring to Point A1) that is close to the reference generally acceptable in the trade (−10 dB). According to the experimental data of Smith Impedance of the flat panel antenna as given in FIG. 3, Curve B indicates impedance covered by the flat panel antenna with the impedance of the central frequency of the antenna staying far away from 1 (referring to Point B1), i.e., the impedance of the antenna is far away from 50 Ω of the system impedance. Both of return loss and impedance of the flat panel antenna of the prior art though meet the criteria generally acceptable in the trade, fail to comply with strict requirements of an antenna with exact specifications, e.g., the one for the substrate station or Radio Frequency Identification(RFID) READER. Furthermore, the radiation efficiency of the flat panel antenna of the prior art is also comparatively poor due to its greater impedance.